Weld-on cast carbon steel roll crusher tooth having silver-brazed tungsten carbide inserts on wear surfaces

ABSTRACT

A durable, weld-on tooth for roll crushers is provided. The tooth includes a cast high-carbon steel body having multiple transverse grooves for receiving tungsten carbide inserts, which are silver brazed into the grooves. The tungsten inserts are located on the surfaces of the tooth that are most susceptible to wear from abrasive action of the coal as the roll crusher operates. The cast carbon steel body can be directly welded to the exterior cylindrical surface of a rotary roll crusher. For a preferred embodiment of the tooth, the front and side faces of the cast high-carbon steel body have bevels that enable weldments that secure it to the roll crusher drum to be somewhat recessed beneath the cast high-carbon steel body on three sides thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to teeth for roll crushers and, moreparticularly, to teeth which are welded to the outer surface of rollcrusher drums.

2. Description of the Prior Art

Roll crushers are compression type crushers widely used extensively inmining operations. There are two basic types of roll crushers. The firstemploys a single roll operating adjacent a curved stationary anvilplate. The second employs two counter-rotating rolls having parallelaxes and a gap between the rolls. Particle output size is determined, inthe case of a single roll crusher, by the gap between the roll and theanvil plate or, in the case of a double-roll crusher, by the gap betweenthe rolls. During the operation of a roll crusher, large particles aredrawn into the gap by the rotating motion of the roll or rolls and afriction, or nip, angle formed between either the single roll and itsadjacent anvil plate or between the two rolls of the double-roll crusherand the particle. As the large particles are forced into an ever smallergap, compressive forces fracture the particles. Roll crushers have atheoretical maximum reduction ratio of 4:1. Thus, if an 8-inch diameterparticle is fed to the roll crusher the absolute smallest size one couldexpect from the crusher is a 2-inch diameter particle.

Though once widely used to crush mined mineral-ore-containing rock, theuse of roll crushers in that application has declined during the pastdecade as low-cost, low-maintenance cone crushers have largely takenover the task. However, because the output from roll crushers has a verynarrow size distribution and very little dust or fines is producedduring the crushing process, roll crushers are still widely used in coalmining operations. Whereas roll crushers used to crush mineral and metalores have smooth faced rolls, those used for crushing coal have teeth orother topography attached to the rolls.

If a coal seam is not too far beneath the surface, the coal is mosteasily mined with the greatest percentage of coal recovery by removingthe overburden to expose the coal seam and, then, blasting and removingthe coal. This is known as surface mining. Surface mining of coal hasbecome widespread where coal seams are relatively close to the surface.The ratio of overburden excavated to the amount of coal removed iscalled the overburden ratio. The lower the ratio, the more productivethe mine. The lowest overburden ratios are found in western surfacemines. In Appalachia, often more than one coal seam is mined.

There are several types of surface coal mines. Area surface mines,usually found in flat terrain, consist of a series of cuts 100 to 200feet wide. The overburden from one cut is used to fill in the mined outarea of the preceding cut. Contour mining, occurring in mountainousterrain, follows a coal seam along the side of the hill. When contourmining becomes too expensive, additional coal can often be produced fromthe mine's highwall by the use of augers or highwall miners. Open pitmines are usually found where coal seams are thick, and can reach depthsof several hundred feet.

Equipment used in surface mines include draglines, shovels, bulldozers,front-end loaders, bucket wheel excavators and trucks. In large mines,draglines remove the overburden while shovels are used to load the coal.In smaller mines, bulldozers and front-end loaders are often used toremove overburden. However, when it coal seam is too far beneath thesurface to make surface mining practical, underground mining is used.

If it is not practical to remove the overburden covering a coal seam,the seam must be mined using underground mining methods. Mostunderground coal is mined by the room and pillar method, whereby roomsare cut into the coal bed leaving a series of pillars, or columns ofcoal, to help support the mine roof and control the flow of air.Generally, rooms are 20-30 feet wide and the pillars up to 100 feetwide. As mining advances, a grid-like pattern of rooms and pillars isformed. When mining advances to the end of a panel or the property line,retreat mining begins. In retreat mining, the workers mine as much coalas possible from the remaining pillars until the roof falls in. Whenretreat mining is completed, the mined area is abandoned. There are twomethods to extract the coal using room and pillar mining: conventionalmining and continuous mining. Conventional mining is the oldest method,and now accounts for only about 12% of underground coal output. Inconventional mining, the coal seam is cut, drilled, blasted and thenloaded into cars. Continuous mining is now the most prevalent form ofunderground mining, accounting for about 56% of total undergroundproduction. In continuous mining, a machine known as a continuous minercuts the coal from the mining face, obviating the need for drilling andblasting.

The longwall method of underground coal mining, which was implementedduring the latter half of the twentieth century, is generally consideredto represent the most revolutionary advance in coal mining technology inhistory. Longwall mining now accounts for about 31% of underground coalproduction. There are about 100 longwall operations in the UnitedStates, with most of them being in Appalachia. In longwall mining, acutting head moves back and forth across a panel of coal about 800 feetin width and up to 7,000 feet in length. The cut coal falls onto aflexible conveyor for removal. Longwall mining is done under hydraulicroof supports (shields) that are advanced as the seam is cut. The roofin the mined out areas falls as the shields advance. About ninetypercent of the coal within a seam is recoverable using the method.

Roll crushers are typically used to treat the output of both surfacemines and underground mines so that lumps of the mined coal measure nomore than 5.0 cm (about 2.0 inches) across. This is generally themaximum size that coal-fired power plants are willing to accept. Suchcrushers are generally of the dual-roll type, and are manufactured bycompanies such as Joy Mining Machinery, Inc. and McLanahan Corporation.The crushers typically utilize a rotary drum to which teeth are affixed.U.S. Pat. No. 4,807,820 to Theodore F. Gundlach discloses a segmentalshell for a coal crusher roll. The teeth are clearly visible on thesegmental shell of the drawings.

In the interest of permanently securing the teeth to crusher rolls,teeth are welded to the cylindrical surface of the crusher roll.Although welding the teeth to the roll greatly enhances overalldurability of the roll, replacing worn-out or broken teeth is no simpletask. When the drum is rebuilt, the worn-out teeth must be cut from theouter surface of the drum, and new teeth welded to the drum to replacethose that have been cut off. The process is labor intensive and costly.Clearly, the longer the longer the life expectancy of the attachedteeth, the longer the drum can be productively used, and the less thedowntime required for rebuilding the drum.

Four basic types of teeth are presently manufactured for use on crusherrolls. The first type is a cast steel tooth having hard facing welded onthe wear surface. Each tooth of this type sells for about $15.00. Incontinuous service, such a tooth lasts only about four weeks.

The second type of tooth is a cast steel tooth having tungsten carbidechips welded onto the wear surface. Each tooth of this type sells forabout $35.00. The problem with this type of tooth is that after thetungsten carbide chips are worn off, the tooth becomes rounded and stopscrushing the coal. In continuous service, such a tooth lasts about 12weeks.

The third type of tooth is a cut steel tooth having a welded-on castmild steel bar with tungsten carbide chips cast into the wear face. Thetooth may also include welded-on hard facing. Each such tooth sells forabout $48. This type of tooth suffers from a number of drawbacks: thecasting of tungsten carbide chips is a slow and difficult process,resulting in high manufacturing costs; and when the tungsten carbidechips are worn off, the tooth is, effectively, unusable. In continuousservice, a tooth of this type also lasts about 12 weeks.

The fourth type of tooth is a mild steel tooth having tungsten carbidechips cast into the wear face. The primary problem with this type oftooth is cost, as the casting of tungsten carbide chips is a difficultand slow process. The chips are gravity fed into the molten mild steelas the casting is poured. The process results in the presence of chipsonly on the face of the tooth. Although each tooth of this type sellsfor about $54.00, it lasts only about 12 weeks in continuous service.

-   -   The focus of the present invention is the manufacture of a more        durable tooth that greatly extends the useful life of the rotary        drums used in roll crushers.

SUMMARY OF THE INVENTION

The present invention provides a durable tooth which can be weldeddirectly to the outer cylindrical surface of the drum of a roll crusher.The tooth includes a cast carbon steel body having multiple transversegrooves for receiving tungsten carbide inserts, which are silver brazedinto the grooves. For a preferred embodiment of the tooth, the castcarbon steel body is made of #7018 carbon steel. The silver brazingalloy joins the materials and compensates for the difference in theirexpansion rates. In addition it provides a cushion between theultra-hard tungsten carbide inserts and the hard steel body, whichsoftens impacts and minimizes damage to the inserts. The tungsteninserts are located on the surfaces of the tooth that are mostsusceptible to wear from abrasive action of the coal as the roll crusheroperates. The cast carbon steel body 100 can be directly welded to theexterior cylindrical surface of a rotary roll crusher.

Though the use of metal alloy containing up to 40 percent silver as amolten joining compound is colloquially referred to as silver soldering,the process is more accurately described as silver brazing. In the U.S.,soldering is traditionally defined as the joining of two componentsusing a metal alloy which has a melting point below 800° F. (427° C.).Though silver brazing is similar to soldering, the filler metal has asignificantly different composition. Common silver brazing alloyscontain as much as 60% silver or as little as 20%, with the remaindermade up of metals including copper, zinc, nickel, and tin. Silverbrazing alloys melt at temperatures as low as 1145 F. There are severalgrades of silver solder, and some flow more easily than others (it isthe silver which provides the free-flowing characteristics). Even thoughsilver brazing compounds are very adept at filling small caps throughcapillary action, they should not be relied on to fill large gapsbetween joined components, as joint strength drops off rapidly as thegap between the joined components increases. As a consequence of thisreality, the gaps between two components of any silver-brazed jointshould be no greater than 0.125 mm (about 0.005 inch) and, ideally, nogreater than half that amount. Such tolerances are typically achievedthrough surface grinding of the mating surfaces. In addition,interference fits between components to be joined must be scrupulouslyavoided, as capillary action will be hindered, and joint integrity willbe significantly damaged. The silver alloy filler compound is broughtslightly above its melting (liquidus) temperature while protected by asuitable atmosphere or flux. It then interacts with a thin layer of thebase metal (known as wetting) and is then cooled rapidly to form asealed joint. By definition, the melting temperature of the braze alloyis lower—often, substantially lower—than the melting temperature of thematerials being joined. Brazed joints are generally stronger than theindividual metals making up the filler alloy due to both the geometry ofthe joint and the metallurgical bonding that occurs at the interface ofeach base metal component and the filler alloy. At the interface, a verythin matrix of filler metal atoms and base metal atoms is formed. Inorder to maximize the strength of brazed joints, base metal parts mustbe exceptionally clean and free of oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the cast carbon steel roll crusher toothbefore installation of the beveled tungsten carbide bars within thegrooves;

FIG. 2 is an isometric view of a single beveled tungsten carbide bar;

FIG. 3 is an isometric view of the completed cast carbon steel rollcrusher tooth having multiple beveled tungsten carbide bars silversoldered to grooves in the wear faces thereof;

FIG. 4 is a front elevational view of the completed cast carbon steelroll crusher tooth;

FIG. 5 is a side elevational view of the completed cast carbon steelroll crusher tooth;

FIG. 6 is a top plan view of the completed cast carbon steel rollcrusher tooth;

FIG. 7 is a bottom plan view of the completed cast carbon steel rollcrusher tooth; and

FIG. 8 is a rear elevational view of the completed cast carbon steelroll crusher tooth.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with reference to the attacheddrawing FIGS. 1 through 8. It should be understood that although thedrawings are intended to be merely illustrative, a reasonable attempthas been made to provide drawings which are close to scale.

Referring now to FIG. 1, a tooth body 100 is cast from #7018 high-carbonsteel. It will be noted that the tooth body 100 has a vertical,generally planar front face 101; a vertical, generally planar right-sideface 102D; an upper surface 103 that has a generally planar forwardportion 103F and a simple-curved rear portion 103R that begins at thelevel of the planar forward portion 103F, and transitions so as to makean acute-angled rear edge 104 with a planar lower surface 701 (pleaserefer to the bottom plan view of FIG. 7). The left-side face 102S(please refer to FIG. 5) is a vertical, generally planar surface and amirror image of the right-side face 102D. For a preferred embodiment ofthe invention, the cast carbon steel body 100 is about 5.1 cm (2.0inches) in height, 3.8 cm (1.5 inches) in width, and 7.6 cm (3.0 inches)in length. The tooth body 100 is equipped with three transverse grooves106A, 106B, and 106C in an upper portion of the front surface 101 andthe planar forward portion 103F of the upper surface 103. Although thegrooves are formed as part of the casting process, they are preferablyprecision ground following the casting process to more exactingtolerances. In addition, the cast high-carbon steel tooth body 100 has20-degree weld bevels 107F, 107D, and 107S on the lower edges of thefront face 101, the right face 102D and the left face 102S,respectively. These bevels, which are about 6.35 mm (¼ inch) in width,allow weldments joining the cast carbon steel body 100 to a roll crusherdrum to be somewhat recessed beneath the cast high-carbon steel body 100around all but the acute-angled rear edge 104 portion of the perimeterthereof.

Referring now to FIG. 2, a single tungsten carbide insert 200 is shown.It will be noted that the insert is a generally rectangular block 201having a crushing face 202 with chamfered edges 203 and a central notch204. Each tungsten carbide insert 200 is manufactured to closetolerances. It will be noted in drawing FIG. 3 through 8 that eachgroove of the carbon steel body holds a single tungsten carbide insert200. A single tungsten carbide insert 200 will be silver brazed intoeach of the three transverse grooves 106A, 106B and 106C in the casthigh-carbon steel body 100. Though the use of metal alloy containing upto 40 percent silver as a molten joining compound is colloquiallyreferred to as silver soldering, the process is more accuratelydescribed as silver brazing. In the U.S., soldering is traditionallydefined as the joining of two components using a metal alloy which has amelting point below 800° F. (427° C.). Though silver brazing is similarto soldering, the filler metal has a significantly different composition(between 20 and 50 percent silver) and a melting temperature above 450°C. (842° F.). There are several grades of silver alloy brazing compound,and some flow more easily than others (it is the silver which providesthe free-flowing characteristics). Even though it is very adept atfilling small caps through capillary action, silver alloy brazingcompound should not be relied on to fill large gaps between joinedcomponents, as joint strength drops off rapidly as the gap between thejoined components increases. As a consequence of this phenomenon, thegaps between each transverse groove 106A, 106B, 106C and its associatedtungsten carbide insert 100 should be no greater than 0.125 mm (about0.005 inch) and, ideally, no greater than half that amount. Thesetolerances are achieved by precision grinding both the transversegrooves 106A, 106B and 106C, as well as each tungsten carbide insert200. Interference fits between each groove 106A, 106B or 106C and itsassociated tungsten carbide insert 200 must be scrupulously avoided, ascapillary action will be hindered, and joint integrity will besignificantly damaged. The silver alloy filler compound is broughtslightly above its melting (liquidus) temperature while protected by asuitable atmosphere or flux. It then interacts with a thin layer of boththe high-carbon steel body 100 or the tungsten carbide insert 200 of thebase metal (known as wetting) and is then cooled rapidly to form asealed joint. By definition, the melting temperature of the braze alloyis lower—often, substantially lower—than the melting temperature of thematerials being joined. Brazed joints are generally stronger than theindividual metals making up the filler alloy due to both the geometry ofthe joint and the metallurgical bonding that occurs at the interface ofeach base metal component and the filler alloy. At the interface, a verythin matrix of filler metal atoms and base metal atoms is formed. Inorder to maximize the strength of brazed joints, base metal parts mustbe exceptionally clean and free of oxide.

Referring now to FIGS. 3 through 8, the high-carbon steel body 100 andthree tungsten carbide inserts 200-A, 200-B and 200-C have been joinedas a single unit.

The new cast carbon steel tooth having beveled tungsten carbide insertssilver brazed in grooves within the wear faces is projected to have aunit price of about $64.00 and last about 26 weeks in continuousservice. This represents about a 33 percent increase in cost and about a100 percent increase in durability compared to the prior art cut steeltooth having the mild steel bar with tungsten carbide chips cast intothe wear face is a mild steel tooth having tungsten carbide chips castinto the wear face. Compared to the mild steel tooth having cast-intungsten carbide chips in the wear face, the cast carbon steel toothhaving beveled tungsten carbide inserts silver soldered to the wear faceis about 19 percent most costly, but about twice as durable.

Although only a single embodiment of the invention has been shown anddescribed, it will be obvious to those having ordinary skill in the artthat changes and modifications may be made thereto without departingfrom the scope and the spirit of the invention.

1. A wear-resistant tooth weldable to a drum of a roll crusher, saidtooth comprising: a carbon steel body having a plurality of transversegrooves on surfaces thereof most susceptible to wear; and a tungstencarbide insert installed and silver brazed in each transverse groove,each carbide insert extending an entire width of said cast carbon steelbody.
 2. The wear-resistant tooth of claim 1, wherein said carbon steelbody is cast from #7018 carbon steel.
 3. The wear-resistant tooth ofclaim 1, wherein each transverse groove is cast into said carbon steelbody.
 4. The wear-resistant tooth of claim 3, wherein each transversegroove is precision ground, following casting of said body, for anoptimum fit with its associated tungsten carbide insert, so thatclearances between said tungsten carbide insert and each transversegroove are within a range of about 0.06 mm to 0.125 mm.
 5. Thewear-resistant tooth of claim 1, wherein said carbon steel body is about5.1 cm in height, 3.8 cm in width, and 7.6 cm in length.
 6. Thewear-resistant tooth of claim 1, wherein said said carbon steel body hasgenerally vertical front and side faces, a generally planar lowersurface, and an upper surface which tapers to make an acute-angledtransverse rear edge with said lower surface.
 7. The wear-resistanttooth of claim 6, wherein each of said faces is equipped with a beveladjacent said lower surface which permits weldments to a roll crusherdrum that are at least partially recessed.
 8. The wear-resistant toothof claim 7, wherein each of said bevels is about 6.5 mm in width andmade an angle of about 20 degrees from each vertical face.
 9. Thewear-resistant tooth of claim 1, wherein said plurality of transversegrooves numbers three.
 10. The wear-resistant tooth of claim 6, whereinsaid plurality of transverse grooves numbers three, a first transversegroove is made on said front face, a second transverse groove is made atan intersection of saif front face and said upper surface, and a thirdtransverse groove is made on said upper surface.
 11. The wear-resistanttooth of claim 6, wherein said upper surface comprises a generallyforward planar portion and a simple-curved rear portion.
 12. Awear-resistant tooth weldable to a drum of a roll crusher, said toothcomprising: a carbon steel body having a generally planar vertical frontface, generally planar side faces, a generally planar lower surface, andan upper surface perpendicular to said side faces which tapers to makean acute angled edge with said lower surface, said carbon steel bodyalso having a plurality of transverse grooves on surfaces thereof mostsusceptible to wear; and a tungsten carbide insert installed and silverbrazed in each transverse groove, each carbide insert extending anentire width of said cast carbon steel body.
 13. The wear-resistanttooth of claim 12, wherein said carbon steel body is cast from #7018carbon steel.
 14. The wear-resistant tooth of claim 12, wherein eachtransverse groove is cast into said carbon steel body.
 15. Thewear-resistant tooth of claim 14, wherein each transverse groove isprecision ground, following casting of said body, for an optimum fitwith its associated tungsten carbide insert.
 16. The wear-resistanttooth of claim 12, wherein said carbon steel body is about 5.1 cm inheight, 3.8 cm in width, and 7.6 cm in length.
 17. The wear-resistanttooth of claim 12, wherein said said carbon steel body has generallyvertical front and side faces, a generally planar lower surface, and anupper surface which tapers to make an acute-angled transverse rear edgewith said lower surface.
 18. The wear-resistant tooth of claim 17,wherein each of said faces is equipped with a bevel adjacent said lowersurface which permits weldments to a roll crusher drum that are at leastpartially recessed.
 19. The wear-resistant tooth of claim 12, whereinsaid plurality of transverse grooves numbers three, and a firsttransverse groove is made on said front face, a second transverse grooveis made at an intersection of saif front face and said upper surface,and a third transverse groove is made on said upper surface.
 20. Thewear-resistant tooth of claim 17, wherein said upper surface comprises agenerally forward planar portion and a simple-curved rear portion.